The technology for injecting calcium-based sorbents directly into furnaces to contact high-temperature combustion gases to remove SO.sub.x has employed the use of a variety of reagents. The most preferred are those which are effective even after being stored and transported for extended periods and can be stored and transported with minimal special handling. Calcium carbonate (commercially available in a number of forms including limestone), calcium oxide, and calcium hydroxide are the materials of choice. However, they tend to plug supply lines, clog spray nozzles, and separate (water from the suspended solids) upon standing.
In U.S. Pat. No. 4,555,996, Torbov discloses that injection of slurries of calcium compounds into furnaces at temperatures above about 2200.degree. F., is superior to the use of the same compounds in dry form. However, Torbov does not identify the problems associated with slurry stability.
In European published patent application No. 373,351, DeMichele and Quattroni discuss the use of a mixture of urea, hydrate lime and water in conjunction with an electrostatic precipitator to reduce nitric oxide and sulfur anhydrides. It is indicated that, when the lime concentration is greater than or equal to 35% of the dispersion, a polysaccharide and ligninsulfonate dispersing agent is employed. However, the dispersing agents are not as effective as desired, and DeMichele and Quattroni do not even recognize the need for one at concentrations of less than 35%. And, while in U.S. Pat. No. 4,869,885, Lin and Mouche' disclose the use of dispersants for wet and dry scrubbers to improve scrubbing efficiency, the reasons addressed there are not the ones of principal concern for in-furnace injection. Again, in-furnace injection uses smaller conduits and nozzles, and injects smaller slurry volumes--typically horizontally--across greater distances into much higher temperature zones. The Lin and Mouche' patent simply does not address the problems of slurry stability under the extreme conditions posed by in-furnace injection.
In U.S. Pat. No. 5,058,514, Mozes, Mangal, and Thampi disclose a slurry injection process for SO.sub.2 and NO.sub.x reduction using calcium carbonate and a nitrogenous progenitor such as urea. No interaction was found. The disclosed advantage is the independent activity of the components--i.e., no loss of activity as compared with independent use of the components. Also, other than noting that maintenance of slurry stability can be achieved by constant recirculation and mixing, the stability of the slurry in lines and nozzles is not addressed.
The prior art does not address the points necessary to achieve improvements in the reliable handling of calcium-based sorbent slurries for infurnace injection for SO.sub.x reduction.